splat.data: refactor sample types

[gctucker]

• remove sample_type and sample_width definitions and use plain strings to select any supported sample type
• update all import/export functions to accept a string instead of a type/width integers combination
• expose native sample type and width
• list all supported types in a dictionary which maps to the width in bits of each sample type
• import _splat.sample_types, _splat.SAMPLE_TYPE and _splat.SAMPLE_WIDTH in splat/init.py
• update tests with the new definitions
• update documentation